Desktop and hi-fi speakers, what’s the difference?

The PSB Alpha B1 speakers on the left are functionally very different from the Audioengine A5+ speakers on the right.
(Credit: Steve Guttenberg/CNET)
Judging by the number of e-mails I get on this subject, a lot of folks don’t understand the difference between computer and hi-fi speakers. For example, how is an Audioengine A5+ ($399 a pair) functionally different from a PSB Alpha B1 speaker ($300 a pair)?
Both have 5-inch woofers and tweeters, but the A5+ has something you won’t see on the Alpha B1, or any hi-fi speaker: a volume control. That’s because the A5+ is a “powered” speaker,…